OnLine 5.0 ESQL/C Query timing problems

We currently have a problem with an ESQL/C query running on an INFORMIX OnLine database version 5.01.UD2. This particular query is run every day at about 6:45 pm. Some days it takes 12 seconds to run, other days it takes about 120 seconds. An update statistics is run every night at 1:00 am. Nobody else is on the machine, and no functions on the system lock that table (although inserts are being done to it constantly). The system is processing messages and this message has a 30 second time out. On the days that it takes 120 odd seconds, the system performance is adversely affected as 4 or 5 of these transactions are processed, tieing up the machine for 8 to 10 minutes. An attempt to add another index to improve the query resulted in it taking 11 minutes !!! This index was then dropped. Running the same query from the command line always takes about 8 seconds. We cannot reproduce the problem on our development system, even when using the same data. It only occurs on the customers machine. Has anybody got any ideas on where to look or what to try to resolve this ?